[en] The certainty and trustworthiness of a planning model can best be assessed when projections can be compared with actual developments. The ability to design scenarios and to evaluate demographic, economic and technological change is also increased with periodic comparisons between plans and actual facts. In 1976, the Mexican government published a 25 year power supply plan for the country, in which a development of non-fossil fuel plants was recommended, largely due to environmental considerations. It was proposed that the new demand should be met mainly with renewable energy sources and nuclear power. The study stated that hydrocarbons would reach a peak between 2003 and 2005, and that after this point the descent of fuel consumption would proceed at an increased velocity. Under this program, the dependence of Mexico on fuel for electricity would be gradually reduced as the 21st century progressed. The suggestion was not assumed by successive governments, and fossil fuel plants took the place of the proposed nuclear plants. However, the original 25 years power plan has proved to be quite accurate in its social and economic previsions, and allows the validation of a new, more powerful and reliable planning model. This paper presents the results of the validation of the model, as well as major considerations to be heeded in the future to increase certainty in further planning efforts. The projection of a better, more sustainable and reliable energy future is also proposed